Wire-based metallization for solar cells

ABSTRACT

Approaches for fabricating wire-based metallization for solar cells, and the resulting solar cells, are described. In an example, a solar cell includes a substrate having a back surface and an opposing light-receiving surface. A plurality of alternating N-type and P-type semiconductor regions is disposed in or above the back surface of the substrate. A conductive contact structure is disposed on the plurality of alternating N-type and P-type semiconductor regions. The conductive contact structure includes a plurality of metal wires. Each metal wire of the plurality of metal wires is parallel along a first direction to form a one-dimensional layout of a metallization layer for the solar cell.

TECHNICAL FIELD

Embodiments of the present disclosure are in the field of renewableenergy and, in particular, include approaches for fabricating wire-basedmetallization for solar cells, and the resulting solar cells.

BACKGROUND

Photovoltaic cells, commonly known as solar cells, are well knowndevices for direct conversion of solar radiation into electrical energy.Generally, solar cells are fabricated on a semiconductor wafer orsubstrate using semiconductor processing techniques to form a p-njunction near a surface of the substrate. Solar radiation impinging onthe surface of, and entering into, the substrate creates electron andhole pairs in the bulk of the substrate. The electron and hole pairsmigrate to p-doped and n-doped regions in the substrate, therebygenerating a voltage differential between the doped regions. The dopedregions are connected to conductive regions on the solar cell to directan electrical current from the cell to an external circuit coupledthereto.

Efficiency is an important characteristic of a solar cell as it isdirectly related to the capability of the solar cell to generate power.Likewise, efficiency in producing solar cells is directly related to thecost effectiveness of such solar cells. Accordingly, techniques forincreasing the efficiency of solar cells, or techniques for increasingthe efficiency in the manufacture of solar cells, are generallydesirable. Some embodiments of the present disclosure allow forincreased solar cell manufacture efficiency by providing novel processesfor fabricating solar cell structures. Some embodiments of the presentdisclosure allow for increased solar cell efficiency by providing novelsolar cell structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a geometrical consideration for strain relief, inaccordance with an embodiment of the present disclosure.

FIG. 2 illustrates plan view of the back side of a solar cell havingwire-based metallization, and the corresponding cross-sectional view, inaccordance with an embodiment of the present disclosure.

FIG. 3 illustrates plan view of the back side of another solar cellhaving wire-based metallization, and the corresponding cross-sectionalview, in accordance with another embodiment of the present disclosure.

FIG. 4 illustrates the metallization arrangement of FIG. 3, as depictedat (a) a bonding temperature and (b) room temperature, in accordancewith an embodiment of the present disclosure.

FIG. 5 illustrates plan view of the back side of a solar cell havingwire-based mesh metallization, in accordance with an embodiment of thepresent disclosure.

FIG. 6 illustrates plan view of the back side of another solar cellhaving wire-based mesh metallization, in accordance with anotherembodiment of the present disclosure.

FIGS. 7A-7C illustrate plan views of the back sides of solar cellshaving wire-based mesh metallization, in accordance with an embodimentof the present disclosure.

FIG. 8 illustrates a photovoltaic assembly having a mesh metallizationstructure, in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. As used herein, the word“exemplary” means “serving as an example, instance, or illustration.”Any implementation described herein as exemplary is not necessarily tobe construed as preferred or advantageous over other implementations.Furthermore, there is no intention to be bound by any expressed orimplied theory presented in the preceding technical field, background,brief summary or the following detailed description.

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Terminology. The following paragraphs provide definitions and/or contextfor terms found in this disclosure (including the appended claims):

“Comprising.” This term is open-ended. As used in the appended claims,this term does not foreclose additional structure or steps.

“Configured To.” Various units or components may be described or claimedas “configured to” perform a task or tasks. In such contexts,“configured to” is used to connote structure by indicating that theunits/components include structure that performs those task or tasksduring operation. As such, the unit/component can be said to beconfigured to perform the task even when the specified unit/component isnot currently operational (e.g., is not on/active). Reciting that aunit/circuit/component is “configured to” perform one or more tasks isexpressly intended not to invoke 35 U.S.C. § 112, sixth paragraph, forthat unit/component.

“First,” “Second,” etc. As used herein, these terms are used as labelsfor nouns that they precede, and do not imply any type of ordering(e.g., spatial, temporal, logical, etc.). For example, reference to a“first” solar cell does not necessarily imply that this solar cell isthe first solar cell in a sequence; instead the term “first” is used todifferentiate this solar cell from another solar cell (e.g., a “second”solar cell).

“Coupled”—The following description refers to elements or nodes orfeatures being “coupled” together. As used herein, unless expresslystated otherwise, “coupled” means that one element/node/feature isdirectly or indirectly joined to (or directly or indirectly communicateswith) another element/node/feature, and not necessarily mechanically.

In addition, certain terminology may also be used in the followingdescription for the purpose of reference only, and thus are not intendedto be limiting. For example, terms such as “upper”, “lower”, “above”,and “below” refer to directions in the drawings to which reference ismade. Terms such as “front”, “back”, “rear”, “side”, “outboard”, and“inboard” describe the orientation and/or location of portions of thecomponent within a consistent but arbitrary frame of reference which ismade clear by reference to the text and the associated drawingsdescribing the component under discussion. Such terminology may includethe words specifically mentioned above, derivatives thereof, and wordsof similar import.

“Inhibit”—As used herein, inhibit is used to describe a reducing orminimizing effect. When a component or feature is described asinhibiting an action, motion, or condition it may completely prevent theresult or outcome or future state completely. Additionally, “inhibit”can also refer to a reduction or lessening of the outcome, performance,and/or effect which might otherwise occur. Accordingly, when acomponent, element, or feature is referred to as inhibiting a result orstate, it need not completely prevent or eliminate the result or state.

Approaches for fabricating wire-based metallization for solar cells, andthe resulting solar cells, are described herein. In the followingdescription, numerous specific details are set forth, such as specificpaste compositions and process flow operations, in order to provide athorough understanding of embodiments of the present disclosure. It willbe apparent to one skilled in the art that embodiments of the presentdisclosure may be practiced without these specific details. In otherinstances, well-known fabrication techniques, such as lithography andpatterning techniques, are not described in detail in order to notunnecessarily obscure embodiments of the present disclosure.Furthermore, it is to be understood that the various embodiments shownin the figures are illustrative representations and are not necessarilydrawn to scale.

Disclosed herein are solar cells. In one embodiment, a solar cellincludes a substrate having a back surface and an opposinglight-receiving surface. A plurality of alternating N-type and P-typesemiconductor regions is disposed in or above the back surface of thesubstrate. A conductive contact structure is disposed on the pluralityof alternating N-type and P-type semiconductor regions. The conductivecontact structure includes a plurality of metal wires. Each metal wireof the plurality of metal wires is parallel along a first direction toform a one-dimensional layout of a metallization layer for the solarcell.

Also disclosed herein are methods of fabricating a solar cell. In anembodiment, a method of fabricating a solar cell includes forming aplurality of alternating N-type and P-type semiconductor regions in orabove a back surface of a substrate, opposite a light-receiving surfaceof the substrate. The method also includes forming a metal seed layer onthe plurality of alternating N-type and P-type semiconductor regions.The method also includes electrically connecting a plurality of metalwires to the metal seed layer. Each metal wire of the plurality of metalwires is parallel along a first direction to form a one-dimensionallayout of a metallization layer for the solar cell.

Also disclosed herein are photovoltaic assemblies. In one embodiment, aphotovoltaic assembly includes a plurality of substrates. Each substratehas a back surface and an opposing light-receiving surface. A pluralityof alternating N-type and P-type semiconductor regions is disposed in orabove the back surface of each of the plurality of substrates. Aconductive contact structure is disposed on the plurality of alternatingN-type and P-type semiconductor regions of each of the substrates. Theconductive contact structure includes a plurality of metal wires. Eachmetal wire of the plurality of metal wires is parallel along a firstdirection to form a one-dimensional layout of a metallization layer foreach of the substrates. One or more wires of the plurality of metalwires is common to two or more of the plurality of substrates.

One or more embodiments described herein are directed to wire basedmetallization of solar cells. An advantage of devices and methodsdescribed herein is the enablement of more cost effective solar cellmanufacturing. For example, the disclosed devices and methods to formsuch devices do not involve the use of electroplating to achieve arequired grid conductivity. Instead, in an embodiment, the discloseddevices and methods to form such devices involve the use of structuredwires of sufficient conductivity. The structured wires are then bondedto an existing metallization layer on the solar cell. In accordance withone or more embodiments described herein, the devices and methodsdescribed below focus on the mechanical structuring of the wire suchthat a high density of interconnects can be achieved. Also, in oneembodiment, a degree of strain relief can be incorporated into the wiregrid.

To provide context, then, embodiments described herein provide a morecost effective cell metallization process by removing an electroplatingoperation from the process flow. Furthermore, there is also a possibleefficiency benefit by removing the need for fabrication of bonding padson the cell.

To provide further context, it is to be appreciated that the first oncell metallization layers (M1) are typically formed by sputtering orevaporation. Such M1 layers are relatively expensive vacuum basedprocesses, and the thickness of such layers is limited to less thanabout 3 microns to prevent wafer bowing during a subsequent forming gasanneal process. Furthermore, the associated cost of deposition andpatterning of M1 increases with the thickness of M1. Therefore, it maybe advantageous for M1 to be fabricated as thin as possible, and yetenable a low resistance metal-semiconductor contact. However, theresistance of a thin (e.g., less than 3 micron) M1 may be too high toefficiently extract current from the interior of the cell to the edges.Accordingly, a second metallization (M2) layer is often incorporatedonto the solar cell. Electroplated metal features or metal foil basedfeatures have been used in the past. By contrast, in accordance withembodiments described herein, a plurality of wires is implementedeffectively as an M2 layer.

In accordance with an embodiment of the present disclosure, the mostefficient geometry for M1 is a plurality of parallel lines, spaced asclose together as process design rules allow. The most efficientgeometry for M2, then, is also a plurality of parallel lines with thesame pitch and parallel to M1. Such an arrangement provides the shortestcurrent path to the appropriate edge of the wafer. An alternativegeometry is where the conductive M2 lines run perpendicular to M1 andare interconnected such that adjacent M2 lines are connected only toN-type or to P-type contacts, respectively, on the wafer.

In accordance with one or more embodiments described herein, a solder isused to bond a wire (M2) to a thin M1 layer. It is to be appreciated,however, that if a straight wire is soldered at once to M1, the thermalmismatch between an underlying silicon substrate or layer and M2 mayinduce a bowing in the cell as it cools down from soldering temperature.To avoid cell bowing, in an embodiment, the wire is formed and solderedin a way such that the M2 is permitted to contract as it cools, withoutinducing bending in the cell. In one approach, strain relief featuresare included in the wires to address such issues, as is described ingreater detail below. Alternatively, in another embodiment, M2 is bondedto M1 without heating the entire assembly, e.g., by laser soldering orlaser welding, and M2 (the plurality of wires) can be a plurality ofstraight wire without strain relief features across the cell. In anembodiment, the cross section of the wire (e.g., the wire end) is roundor square. In the latter embodiment, a benefit of such a square wire isto provide more contact area between the M1 and M2 layers.

In accordance with one or more embodiments of the present disclosure,strain relief features are incorporated into a solar cell metallizationstructure based on wires. Two approaches may be considered for wirebased strain relief: (1) in-plane strain relief and (2) out-of-plane. Inan embodiment, both approaches involve M1 to M2 bonding at multiplediscrete points along the wire. Factors for consideration include thebonding temperature and the length of wire contained between twoadjacent bonding points, which is longer than the straight linedistance. When the structure is cooled after bonding, the bending forceof the wafer is reduced. Furthermore, the strain on the bonds and waferdue to thermal cycling in normal solar cell operation is reduced by thestrain relief.

In an exemplary embodiment that accounts for strain relief, to calculatethe amount of deformation required, it is considered that copper has alinear coefficient of thermal expansion (CTE) of 17E-6. It is assumedthat a solder connection solidifies at approximately 250 degreesCelsius. Cooling from 250 degrees Celsius to 20 degrees Celsius afterbonding leads to a factor of contraction of 0.00391 of the originallength. FIG. 1 illustrates a geometrical consideration for strainrelief, in accordance with an embodiment of the present disclosure.Referring to FIG. 1, segment 102 represents half the portion of a wire102/102′ at 20 degrees Celsius. Segment 104 represents half the wire104/104′ at the bond temperature for a 2 mm bond spacing (i.e., betweenbond points 106). At 250 degrees Celsius, the length of segment 104 is1.00391×1000 microns which equals 1004 microns. The height 108 of 88microns represents the minimum amount of bending required such that thewire (102/102′ or 104/104′) may contract without placing any strain inthe bond points 106. For a 250 micron wide M1 finger, then, in-planestrain relief may be realized by making small bends in the wire with anamplitude less than the half-width of the M1 finger width, as shown inFIG. 2, described below. Alternatively, the wire may be bent out of theplane on the wafer as shown in FIGS. 3 and 4, described below.

As a first example of wire based metallization for a solar cell, FIG. 2illustrates plan view of the back side of a solar cell having wire-basedmetallization, and the corresponding cross-sectional view, in accordancewith an embodiment of the present disclosure.

Referring to FIG. 2, a portion 200 of a solar cell includes a substrate202 having a back surface 204 and an opposing light-receiving surface206. A plurality of alternating N-type and P-type semiconductor regions(one such regions shown as 208) is disposed in or above the back surface204 of the substrate 202. A conductive contact structure is disposed onthe plurality of alternating N-type and P-type semiconductor regions208. The conductive contact structure includes a plurality of metalwires (one metal wire shown as 210). Each metal wire 210 is bonded tothe solar cell at bonding points 213, which may be solder bonds. Eachmetal wire 210 of the plurality of metal wires is parallel along a firstdirection 212 to form a one-dimensional layout of a metallization layerfor the solar cell 200, examples of which are described in greaterdetail below in association with FIGS. 5 and 6. It is to be appreciatedthat instead of solder bonds, other bonding approaches may be used suchas, but not limited to, thermo-compression bonding, laser welding, orultrasonic assisted welding.

Referring again to FIG. 2, in an embodiment, each metal wire 210 of theplurality of metal wires is undulating in a plane parallel with the backsurface 204 of the substrate 200. In one such embodiment, the undulatinggeometry in the plane parallel with the back surface 204 provides astress relief feature for the solar cell. In an embodiment, as isdepicted in FIG. 2, the conductive contact structure further includes ametal seed layer 214 (i.e., an M1 layer) disposed between the pluralityof alternating N-type and P-type semiconductor regions 208 and theplurality of metal wires 210.

As a second example of wire based metallization for a solar cell, FIG. 3illustrates plan view of the back side of another solar cell havingwire-based metallization, and the corresponding cross-sectional view, inaccordance with another embodiment of the present disclosure.

Referring to FIG. 3, a portion 300 of a solar cell includes a substrate302 having a back surface 304 and an opposing light-receiving surface306. A plurality of alternating N-type and P-type semiconductor regions(one such regions shown as 308) is disposed in or above the back surface304 of the substrate 302. A conductive contact structure is disposed onthe plurality of alternating N-type and P-type semiconductor regions308. The conductive contact structure includes a plurality of metalwires (one metal wire shown as 310). Each metal wire 310 is bonded tothe solar cell at bonding points 313, which may be solder bonds. Eachmetal wire 310 of the plurality of metal wires is parallel along a firstdirection 312 to form a one-dimensional layout of a metallization layerfor the solar cell 300, examples of which are described in greaterdetail below in association with FIGS. 5 and 6.

Referring again to FIG. 3, in an embodiment, each metal wire 310 of theplurality of metal wires is undulating in a plane normal to the backsurface 304 of the substrate 300. In one such embodiment, the undulatinggeometry in the plane normal to the back surface 304 provides a stressrelief feature for the solar cell. In an embodiment, as is depicted inFIG. 3, the conductive contact structure further includes a metal seedlayer 314 (i.e., an M1 layer) disposed between the plurality ofalternating N-type and P-type semiconductor regions 308 and theplurality of metal wires 310.

To demonstrate the effects of temperature on the wire geometry of wirebased metallization for a solar cell, FIG. 4 illustrates themetallization arrangement of FIG. 3, as depicted at (a) a bondingtemperature and (b) room temperature, in accordance with an embodimentof the present disclosure. Referring to FIG. 4, the out of planeundulation of the wire 310 is greater at the bonding temperature than atroom temperature. In an exemplary embodiment, bonding temperature isapproximately 250 degrees Celsius.

In another aspect, a metallization structure for a solar cell includeswoven wires. A woven wire configuration can involve the implementationof metal (e.g., Al or Cu) wires and insulating wires, which are woveninto a mesh such that the metal wires can contact the metallization onthe silicon cell in order to realize the lateral conductivity with metalwires. Such an arrangement may be made where the M2 (wire) layer isparallel to the M1 layer, as shown in FIG. 5, or where the M2 layer isperpendicular to M1 as, shown in FIG. 6. The contact to the cell isrealized at the outer points of the metal wires (where they cross aninsulating wire). The insulating wires may provide structural integrityfor fabricating a non-fragile mesh which is easy to handle. In anembodiment, the insulating wires may be retained in the final cellstructure, or may be removed after the mesh has been bonded to thewafer.

As a first example of wire based metallization having a mesh structure,FIG. 5 illustrates plan view of the back side of a solar cell havingwire-based mesh metallization, in accordance with an embodiment of thepresent disclosure.

Referring to FIG. 5, a portion 500 of a solar cell includes a substrate502 having a back surface 504 and an opposing light-receiving surface(not shown). A plurality of alternating N-type and P-type semiconductorregions 508 is disposed in or above the back surface 504 of thesubstrate 502. A conductive contact structure is disposed on theplurality of alternating N-type and P-type semiconductor regions 508.The conductive contact structure includes a metal seed layer 514 (i.e.,an M1 layer) disposed on the plurality of alternating N-type and P-typesemiconductor regions 508. The conductive contact structure alsoincludes a plurality of metal wires 510. Each metal wire of theplurality of metal wires 510 is bonded to the M1 layer of the solar cellat bonding points 513, which may be solder bonds. Each metal wire of theplurality of metal wires 510 is parallel along a first direction 512 toform a one-dimensional layout of a metallization layer for the solarcell 500. In an embodiment, as is depicted in FIG. 5, the plurality ofalternating N-type and P-type semiconductor regions 508 is parallelalong the first direction 512.

Referring again to FIG. 5, the metallization structure further includesa plurality of insulating wires 520. Each insulating wire of theplurality of insulating wires 520 is parallel along a direction 522orthogonal to the first direction 512. In one such embodiment, eachinsulating wire of the plurality of insulating wires 520 is woventhrough the plurality of metal wires, a 1:1 alternating weaving patternfor which is depicted in FIG. 5. In an embodiment, the plurality ofinsulating wires 520 provides structural integrity for the plurality ofmetal wires 510.

As a second example of wire based metallization having a mesh structure,FIG. 6 illustrates plan view of the back side of another solar cellhaving wire-based mesh metallization, in accordance with anotherembodiment of the present disclosure.

Referring to FIG. 6, a portion 600 of a solar cell includes a substrate602 having a back surface 604 and an opposing light-receiving surface(not shown). A plurality of alternating N-type and P-type semiconductorregions 608 is disposed in or above the back surface 604 of thesubstrate 602. A conductive contact structure is disposed on theplurality of alternating N-type and P-type semiconductor regions 608.The conductive contact structure includes a metal seed layer 614 (i.e.,an M1 layer) disposed on the plurality of alternating N-type and P-typesemiconductor regions 608. The conductive contact structure alsoincludes a plurality of metal wires 610. Each metal wire of theplurality of metal wires 610 is bonded to the M1 layer of the solar cellat bonding points 613, which may be solder bonds. Each metal wire of theplurality of metal wires 610 is parallel along a first direction 612 toform a one-dimensional layout of a metallization layer for the solarcell 600. In an embodiment, as is depicted in FIG. 6, the plurality ofalternating N-type and P-type semiconductor regions 608 is orthogonal tothe first direction 612.

Referring again to FIG. 6, the metallization structure further includesa plurality of insulating wires 620. Each insulating wire of theplurality of insulating wires 620 is parallel along a direction 622orthogonal to the first direction 612. In one such embodiment, eachinsulating wire of the plurality of insulating wires 620 is woventhrough the plurality of metal wires, a 1:1 alternating weaving patternfor which is depicted in FIG. 6. In an embodiment, the plurality ofinsulating wires 620 provides structural integrity for the plurality ofmetal wires 610.

In an embodiment, an entire mesh structure is sized to have an areaapproximately the same as the area of a solar cell. However, in one suchembodiment, at the two ends of the metal wires, i.e., as the ends of thesolar cell, the wires of one polarity (i.e., N-type or P-type underlyingregion) are extended to contact the opposite polarity of an adjacentcell. In an embodiment, then, a mesh of the size of entire module of aplurality of solar cells may be implemented. In one such embodiment, onewire lies above one finger with a certain polarity and conducts all thecurrent of that finger. Next to and in parallel to that wire, anothermetal wire conducts the current of the opposite polarity.

In another embodiment, more than one wire contacts one finger. In such acase, the wire diameter can be reduced, and a finer mesh is achieved.The metal wires may not need to be aligned to the finger, but rather mayonly need to be parallel to the fingers. Where the metal wire crosses anisolating finger and where the metal wire faces the silicon wafer, thelatter portion may provide an “outer point” where the wire contacts thesolar cell. In an embodiment, wires that lie between the on-cellmetallization do not conduct current and, hence, the alignment tolerancebetween mesh and cells does not need to be very fine in the directionperpendicular to the metal finger.

As examples of wire based metallization having a mesh structures withalternative weaving arrangements, FIGS. 7A-7C illustrate plan views ofthe back sides of solar cells having wire-based mesh metallization, inaccordance with an embodiment of the present disclosure. Differentstructural arrangements may provide different levels of advantage withrespect to increased contact area between M1 and M2 and/or optimizedstrain relief geometry.

Referring to FIG. 7A, a plurality of metal wires 710A is parallel with aplurality of M1 layers 714A. A plurality of insulating wires 720A isorthogonal to the plurality of metal wires 710A. From metal wire tometal wire within the plurality of metal wires 710A, the weaving of themetal wires is alternating by ones. With respect to the insulating wires720A, the weaving of the metal wires is spaced for every two insulatingwires.

Referring to FIG. 7B, a plurality of metal wires 710B is parallel with aplurality of M1 layers 714B. A plurality of insulating wires 720B isorthogonal to the plurality of metal wires 710B. From metal wire tometal wire within the plurality of metal wires 710B, the weaving of themetal wires is alternating by twos. With respect to the insulating wires720B, the weaving of the metal wires is spaced for every two insulatingwires.

Referring to FIG. 7C, a plurality of metal wires 710C is parallel with aplurality of M1 layers 714C. A plurality of insulating wires 720C isorthogonal to the plurality of metal wires 710C. From metal wire tometal wire within the plurality of metal wires 710C, the weaving of themetal wires is somewhat randomized. With respect to the insulating wires720C, the weaving of the metal wires is also somewhat randomized.

With reference again to FIGS. 2-6 and 7A-7C, then, a method offabricating a solar cell includes forming a plurality of alternatingN-type and P-type semiconductor regions in or above a back surface of asubstrate, opposite a light-receiving surface of the substrate. Themethod also includes forming a metal seed (M1) layer on the plurality ofalternating N-type and P-type semiconductor regions. The method alsoincludes electrically connecting a plurality of metal wires to the metalseed layer. Each metal wire of the plurality of metal wires is parallelalong a first direction to form a one-dimensional layout of ametallization layer for the solar cell.

In an embodiment, the method further involves weaving a plurality ofinsulating wires through the plurality of metal wires. In an embodiment,electrically connecting the plurality of metal wires to the metal seedlayer involves soldering or welding the plurality of metal wires to themetal seed layer at points along each of the metal wires of theplurality of metal wires.

In another aspect, module integration is addressed in greater detail. Inan embodiment, a mesh is run along a full string of solar cells. Asdescribed in greater detail below in association with FIG. 8, in anembodiment, the plurality of metal wires is further coupled to a secondsubstrate. In one such embodiment, one polarity of the plurality ofmetal wires is disconnected between substrates to form a seriesconnection between the substrates. In a particular such embodiment, theseparation of the segments is readily performed by cutting/laseringevery other wire in the gap between two cells. FIG. 8 illustrates aphotovoltaic assembly having a mesh metallization structure, inaccordance with an embodiment of the present disclosure.

Referring to FIG. 8, a photovoltaic assembly 800 includes a plurality ofsubstrates 802A, 802B, 802C. Each substrate has a back surface 804 andan opposing light-receiving surface 806. A plurality of alternatingN-type and P-type semiconductor regions (not shown) is disposed in orabove the back surface 806 of each of the plurality of substrates 802A,802B, 802C. A conductive contact structure is disposed on the pluralityof alternating N-type and P-type semiconductor regions of each of thesubstrates 802A, 802B, 802C. The conductive contact structure includes aplurality of metal wires, e.g., 810A and 810B. Each metal wire of theplurality of metal wires (e.g., wires 802A and 802B) is parallel along afirst direction to form a one-dimensional layout of a metallizationlayer for each of the substrates, which are bonded at points 813. One ormore wires (e.g., wire 802A) of the plurality of metal wires is commonto two or more of the plurality of substrates 802A, 802B, 802C.Additionally, in one embodiment, a plurality of insulating wires 820 isincluded (ends are shown in FIG. 8). Each insulating wire of theplurality of insulating wires 820 is parallel along a directionorthogonal to the first direction. Furthermore, each insulating wire ofthe plurality of insulating wires 820 is woven through the plurality ofmetal wires 810A, 810B.

It is to be appreciate that the separation of the segments is readilyperformed by cutting/lasering every other wire in the gap between twocells. For example, in an embodiment, as is depicted in FIG. 8, allwires on top of the woven insulator wires at a certain line (Cut A), andalternating in the next gap, are cut (Cut B). Thus, a full string couldbe formed at once. In a specific embodiment, a wider insulator is wovenin at that stage, if needed to accommodate for the cutting, or as aseparator between cells.

In accordance with one or more embodiments described herein, advantagesof implementations described herein include the use of a low cost (andlightweight) metal material as a lateral conductor. By comparison, toachieve the same conductivity as a plated copper finger, an aluminumwire would need to only have a diameter of approximately 100 to 200microns. Other advantages include that the M2 is already patterned, andthe wires are isolated from one other and are formed into lines. In adirection along the fingers the alignment of M1 to M2 need not be veryaccurate, since the wires only need to be parallel to fingers. If themetal finger width of M1 is about 250 microns and the Al wire connectsthe cell only on small areas, the wire does not need to be in the middleof the finger of M1, providing a high alignment tolerance. Furthermore,the bending in the metal wires provides a stress relief element whichimproves reliability of the module, especially with respect to thermalcycling and module bending. In an embodiment, a solar cell is thusprovided where the majority of the current is extracted from the cellvia a wire structure which has been formed in such a way so as to affordmechanical strain relief between discrete electrical bonding points tothe wafer.

In an embodiment, conductive wires as described herein are formed ofcopper or aluminum, with or without a coating such as tin, silver,nickel or an organic solderability protectant. In an embodiment, thesurface of each wire is mostly oxidized, with some regions where theoxide has been removed to enable solder wetting. In one embodiment, thesurface of the wire is mostly coated with an insulating material, withsome regions where the insulating material has been removed to enablesolder wetting. In an embodiment, the wire structure is bonded to thewafer using solder with a melting point less than approximately 300degrees Celsius. In an embodiment, a solder mask material is printedonto the cell to define discrete electrical bonding points. In anembodiment, a substantial portion of the wire is not wet by solder. Inan embodiment, the wire structure is a woven mesh. In one suchembodiment, the weave or weft threads are insulating. In one embodiment,the weave and weft of the wires contact alternate polaritiesrespectively on the wafer.

In an embodiment, a substrate as described herein is a monocrystallinesilicon substrate, such as a bulk single crystalline N-type dopedsilicon substrate. It is to be appreciated, however, that the substratemay be a layer, such as a multi-crystalline silicon layer, disposed on aglobal solar cell substrate.

In an embodiment, alternating N-type and P-type semiconductor regionsdescribed herein are formed from polycrystalline silicon and are formedabove a substrate. In one such embodiment, the N-type polycrystallinesilicon emitter regions are doped with an N-type impurity, such asphosphorus. The P-type polycrystalline silicon emitter regions are dopedwith a P-type impurity, such as boron. The alternating N-type and P-typesemiconductor regions may have trenches formed there between, thetrenches extending partially into the substrate. Additionally, althoughnot depicted, in one embodiment, a bottom anti-reflective coating (BARC)material (also known as a rear dielectric), or other protective layer(such as a layer amorphous silicon) may be formed on the alternatingN-type and P-type semiconductor regions. The alternating N-type andP-type semiconductor regions may be formed on a thin dielectrictunneling layer formed on the back surface of the substrate. In anotherembodiment, alternating N-type and P-type semiconductor regionsdescribed herein are formed as a plurality of N-type and P-typediffusion regions formed in monocrystalline silicon substrate.

In an embodiment, a light receiving surface of a solar cell as describedherein may be a texturized light-receiving surface. In one embodiment, ahydroxide-based wet etchant is employed to texturize the light receivingsurface of the substrate. In an embodiment, a texturized surface may beone which has a regular or an irregular shaped surface for scatteringincoming light, decreasing the amount of light reflected off of thelight receiving surface of the solar cell. Additional embodiments caninclude formation of a passivation and/or anti-reflective coating (ARC)layers on the light-receiving surface.

In an embodiment, an M1 layer, if included, is a plurality of metal seedmaterial regions. In a particular such embodiment, the metal seedmaterial regions are aluminum regions each having a thicknessapproximately in the range of 0.3 to 20 microns and composed of aluminumin an amount greater than approximately 97% and silicon in an amountapproximately in the range of 0-2%. It is to be appreciated thatembodiments described herein involve use of a metal seed (M1) layer.Optionally, in other embodiment, an M1 layer is omitted and the wiresmake direct contact with the silicon.

Although certain materials are described specifically with reference toabove described embodiments, some materials may be readily substitutedwith others with other such embodiments remaining within the spirit andscope of embodiments of the present disclosure. For example, in anembodiment, a different material substrate, such as a group III-Vmaterial substrate, can be used instead of a silicon substrate.Additionally, although reference is made significantly to back contactsolar cell arrangements, it is to be appreciated that approachesdescribed herein may have application to front contact solar cells aswell. In other embodiments, the above described approaches can beapplicable to manufacturing of other than solar cells. For example,manufacturing of light emitting diode (LEDs) may benefit from approachesdescribed herein.

Thus, approaches for fabricating wire-based metallization for solarcells, and the resulting solar cells, have been disclosed.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

What is claimed is:
 1. A solar cell, comprising: a substrate having aback surface and an opposing light-receiving surface; a plurality ofalternating N-type and P-type semiconductor regions disposed in or abovethe back surface of the substrate, each of the plurality of alternatingN-type and P-type semiconductor regions having a length along a firstdirection; and a conductive contact structure disposed on the pluralityof alternating N-type and P-type semiconductor regions, the conductivecontact structure comprising a plurality of metal wires, wherein eachmetal wire of the plurality of metal wires is parallel along the firstdirection to form a one-dimensional layout of a metallization layer forthe solar cell, wherein each metal wire of the plurality of metal wiresis associated with only a single one of the plurality of alternatingN-type and P-type semiconductor regions, and wherein each metal wire ofthe plurality of metal wires is continuously undulating between andextending beyond two bonding points coupling the metal wire to theplurality of alternating N-type and P-type semiconductor regions.
 2. Thesolar cell of claim 1, wherein each metal wire of the plurality of metalwires is undulating in a plane parallel with the back surface of thesubstrate.
 3. The solar cell of claim 1, wherein each metal wire of theplurality of metal wires is undulating in a plane normal to the backsurface of the substrate.
 4. The solar cell of claim 1, wherein eachmetal wire of the plurality of metal wires comprises a stress relieffeature.
 5. The solar cell of claim 1, further comprising: a pluralityof insulating wires, wherein each insulating wire of the plurality ofinsulating wires is parallel along a direction orthogonal to the firstdirection.
 6. The solar cell of claim 5, wherein each insulating wire ofthe plurality of insulating wires is woven through the plurality ofmetal wires.
 7. The solar cell of claim 6, wherein the plurality ofinsulating wires provides structural integrity for the plurality ofmetal wires.
 8. The solar cell of claim 1, wherein the conductivecontact structure further comprises a metal seed layer disposed betweenthe plurality of alternating N-type and P-type semiconductor regions andthe plurality of metal wires.
 9. The solar cell of claim 1, wherein thesubstrate is a monocrystalline silicon substrate, and wherein theplurality of alternating N-type and P-type semiconductor regions is aplurality of N-type and P-type diffusion regions formed in the siliconsubstrate.
 10. The solar cell of claim 1, wherein plurality ofalternating N-type and P-type semiconductor regions is a plurality ofN-type and P-type polycrystalline silicon regions formed above the backsurface of the substrate.
 11. A photovoltaic assembly, comprising: aplurality of substrates, each substrate having a back surface and anopposing light-receiving surface; a plurality of alternating N-type andP-type semiconductor regions disposed in or above the back surface ofeach of the plurality of substrates, each of the plurality ofalternating N-type and P-type semiconductor regions having a lengthalong a first direction; and a conductive contact structure disposed onthe plurality of alternating N-type and P-type semiconductor regions ofeach of the substrates, the conductive contact structure comprising aplurality of metal wires, wherein each metal wire of the plurality ofmetal wires is parallel along the first direction to form aone-dimensional layout of a metallization layer for each of thesubstrates, wherein, for a single substrate each metal wire of theplurality of metal wires is associated with only a single one of theplurality of alternating N-type and P-type semiconductor regions,wherein one or more wires of the plurality of metal wires is common totwo or more of the plurality of substrates, and wherein each metal wireof the plurality of metal wires is continuously undulating between andextending beyond two bonding points coupling the metal wire to theplurality of alternating N-type and P-type semiconductor regions. 12.The photovoltaic assembly of claim 11, wherein each metal wire of theplurality of metal wires comprises a stress relief feature.
 13. Thephotovoltaic assembly of claim 11, further comprising: a plurality ofinsulating wires, wherein each insulating wire of the plurality ofinsulating wires is parallel along a direction orthogonal to the firstdirection, wherein each insulating wire of the plurality of insulatingwires is woven through the plurality of metal wires, and wherein theplurality of insulating wires provides structural integrity for theplurality of metal wires.
 14. The photovoltaic assembly of claim 11,wherein the conductive contact structure further comprises a metal seedlayer disposed between the plurality of alternating N-type and P-typesemiconductor regions of each substrate and the plurality of metalwires.
 15. A method of fabricating a solar cell, the method comprising:forming a plurality of alternating N-type and P-type semiconductorregions in or above a back surface of a substrate, opposite alight-receiving surface of the substrate, each of the plurality ofalternating N-type and P-type semiconductor regions having a lengthalong a first direction; forming a metal seed layer on the plurality ofalternating N-type and P-type semiconductor regions; and electricallyconnecting a plurality of metal wires to the metal seed layer, whereineach metal wire of the plurality of metal wires is parallel along thefirst direction to form a one-dimensional layout of a metallizationlayer for the solar cell, wherein each metal wire of the plurality ofmetal wires is associated with only a single one of the plurality ofalternating N-type and P-type semiconductor regions, and wherein eachmetal wire of the plurality of metal wires is continuously undulatingbetween and extending beyond two bonding points coupling the metal wireto the plurality of alternating N-type and P-type semiconductor regions.16. The method of claim 15, further comprising: weaving a plurality ofinsulating wires through the plurality of metal wires.
 17. The method ofclaim 15, wherein electrically connecting the plurality of metal wiresto the metal seed layer comprises soldering or welding the plurality ofmetal wires to the metal seed layer at points along each of the metalwires of the plurality of metal wires.
 18. The method of claim 15,wherein the plurality of metal wires is further coupled to a secondsubstrate, the method further comprising: disconnecting one polarity ofthe plurality of metal wires between the substrate and the secondsubstrate to form a series connection between the substrate and thesecond substrate.
 19. A solar cell fabricated according to the method ofclaim 15.